A conventional wavelength-selective diffraction element comprises a glass substrate and a dielectric material cyclically formed on the glass substrate. For incident light having a first wavelength, the length of an optical path difference between a part having the dielectric material formed on the glass substrate and a part having no dielectric material is set to a value integer times as large as the first wavelength to transmit the incident light having the first wavelength. For incident light having a second wavelength, the above-described length of the optical path difference is set to a value non-integer times as large as the second wavelength to diffract the incident light having the second wavelength (for instance, see Patent Document 1).
However, in the wavelength-selective diffraction element disclosed in the Patent Document 1, since a condition that the length of the optical path difference is set, for the incident light of the first wavelength, to the value the integer times as large as the first wavelength restricts a degree of freedom in a design for the incident light of the second wavelength, a problem arises that a degree of freedom in selecting a diffraction efficiency is restricted. To solve this problem, a wavelength-selective diffraction element as disclosed in, for instance, Patent Document 2, is proposed.
The wavelength-selective diffraction element disclosed in the Patent Document 2 comprises a transparent substrate on which a grating is so formed as to be cyclically concavo-convex, and a filling member filling the concavo-convex portion of the grating. Either a concavo-convex member forming the concavo-convex part of the grating or the filling member includes an organic pigment having an absorption edge of light in a range of a wavelength shorter than the first wavelength. The concavo-convex member and the filling member have the same refractive index with respect to the light of one of the first wavelength and the second wavelength to transmit the light of one of the first wavelength and the second wavelength without diffracting the light, and have different refractive indices with respect to the light of the other wavelength to diffract the light of the other wavelength. In the Patent Document 2, it is disclosed an organic pigment that has the absorption edge of light in the range of the wavelength of 580 nm to 600 nm.
Patent Document 1: Japanese Patent Publication No. 4-129040A
Patent Document 2: Japanese Patent Publication No. 2002-318306A